1. Field of the Invention
The present invention relates to paste compositions and green sheets each having, for example, photosensitivity, insulation properties or electrical conductivity, and to multilayer substrates obtained by the paste compositions or the green sheets.
2. Description of the Related Art
Demands have been made in recent years to provide miniaturized and high-performance high frequency electronic parts for use in, for example, mobile communication devices, satellite broadcasting reception devices and computers. In addition, wiring patterns of such high frequency electronic parts should be compliant with a higher density of the parts and accelerated signals. To achieve the higher density of the parts and accelerated signals, the wiring patters must become finer and thicker.
The wiring patterns for high frequency electronic parts have been formed by a technique which comprises the steps of mixing an electrically conductive powder containing iron, copper or another polyvalent metal with an organic vehicle including an organic binder and/or an organic solvent to yield a conductive paste, forming a wiring pattern on an insulating substrate with the use of the conductive paste, drying the patterned substrate and firing the dried substrate. Such wiring patterns are generally formed by a screen printing technique, but wiring patterns formed by this technique cannot significantly have a width and pitch less than about 50 xcexcm.
As a possible solution to this problem, for example, Japanese Unexamined Patent Publication No. 5-287221, and Japanese Unexamined Patent Publication No. 8-227153 each propose a technique for the formation of a fine and thick film wiring pattern by a photolithographic technique using a photosensitive conductive paste. In such a photolithographic technique using a photosensitive conductive paste, the development should be preferably performed using water or an alkali from the viewpoint of environmental issues. For this purpose, an acidic functional group is introduced into the organic binder, and from the functional group a proton such as a carboxyl group or a hydroxyl group will be liberated.
When such a photosensitive organic binder is employed and the conductive material is a polyvalent metal, however, it is observed that an anion of the organic binder formed after the liberation of the proton reacts with polyvalent metal ions to form a three-dimensional network through ionic bridging to thereby cause gelation.
Japanese Unexamined Patent Publication No. 9-21850 and Japanese Unexamined Patent Publication No. 9-218508 each state that this problem can be solved by the incorporation of a phosphorus-containing compound such as phosphoric acid into a photosensitive conductive paste, or by the incorporation of an azole-structure compound such as benzotriazole into the photosensitive conductive paste, respectively. These techniques, however, only elongate the gelation time of the photosensitive conductive paste to some extent, and wiring patterns obtained by these techniques cannot be significantly used in practice.
In the aforementioned high frequency circuit components, an electrically insulating layer for isolating two or more electrode patterns or wiring patterns from each other must have a low dielectric constant and a high Q-value to achieve the miniaturization and high performance of the components. Where necessary, the electrically insulating layer includes a hole, i.e., a via hole is formed in such an electrically insulating layer for electrically connecting electrode patterns or wiring patterns formed over and under the layer with each other.
In the formation of such an electrically insulating layer, a process of forming a fine via hole through the photolithographic technique is known, as in the conductive paste. For example, Japanese Unexamined Patent Publication No. 9-110466 and Japanese Unexamined Patent Publication No. 8-50811 each disclose a process comprising the step of dispersing a powdered glass into a photosensitive organic vehicle, which organic vehicle contains an organic binder having a carboxyl group in a side chain to yield a slurry, molding the slurry into a sheet and forming a via hole through the photolithographic technique.
However, as in the photosensitive conductive paste, the anion of the organic binder formed after the liberation of the proton may react with a polyvalent metal ion such as boron or barium eluted from the glass component to form a three-dimensional network through ionic bridging thereby to yield a gel.
To ensure high frequency module substrates, high frequency electronic parts and other multilayer substrates have more functions, higher density and higher performances, it is effective to form high density wiring patterns on the substrates housing a passive component such as a capacitor or a coil.
As a process for fabricating a multilayer substrate housing a variety of passive components and having high density wiring patterns, Japanese Unexamined Patent Publication No. 9-92983 discloses a process comprising the steps of disposing a dielectric ceramic green sheet or a magnetic ceramic green sheet in the form of a sheet inside a multilayer substrate, and partially forming an inductor or a capacitor. This process requires an insulating green sheet obtained by molding a slurry comprising a mixture of a insulating ceramic material and a powdered glass or another inorganic powder with an organic vehicle into a sheet, a magnetic green sheet obtained by molding a slurry comprising a mixture of a magnetic ceramic material and a powdered glass or another inorganic powder with an organic vehicle into a sheet, or a dielectric green sheet obtained by molding a slurry comprising a mixture of a dielectric ceramic material and a powdered glass or another inorganic powder with an organic vehicle into a sheet.
Organic binders in such organic vehicles for use in the above applications often contain an acidic functional group such as a hydroxyl group in the side chain. When a powdered glass containing a polyvalent metal is added to the organic binder of this type, the anion of the organic binder may be ionically bridged with a polyvalent metal ion to cause gelation. If the insulating green sheet, magnetic green sheet, dielectric green sheet or the like is prepared by using a gelled and heterogenous slurry, cracks may be formed during firing and only multilayer substrates having a low reliability could be obtained.
To overcome the above described problems, preferred embodiments of the present invention provide a paste composition which is capable of inhibiting gelation in a mixture of an organic binder having an acidic functional group with a polyvalent metal or a polyvalent metallic compound and is excellent in storage stability and characteristics after firing.
Further, preferred embodiments of the present invention provide a green sheet which is capable of inhibiting gelation in a mixture of an organic binder having an acidic functional group with a glass material or a ceramic material and has an excellent storage stability and a satisfactory reliability after firing.
Further, preferred embodiments of the present invention provide a multilayer substrate which is capable of highly precisely forming a circuit pattern in high density and has an excellent reliability.
After intensive investigations to solve the above problems, the present inventors found that gelation of a system including a mixture of an organic binder having an acidic functional group with a polyvalent metal and/or a polyvalent metallic compound can be effectively inhibited by the incorporation of hydroxyapatite or another anion-adsorptive substance into the system.
Specifically, they found that when hydroxyapatite or another anion-adsorptive substance is added to a system containing an organic binder having an acidic functional group that will release a proton, the anion of the organic binder formed after the liberation of the proton is adsorbed by the anion-adsorptive substance to form a microstructure such as a microphase separation in the mixture; and that this configuration can prevent the formation of a three-dimensional network through ionic bridging even when a polyvalent metal and/or a polyvalent metallic compound is incorporated into the system to thereby inhibit the gelation of the system.
One preferred embodiment of the present invention provides a paste composition (hereinafter referred to as xe2x80x9cthe invented paste compositionxe2x80x9d) including a mixture of:
an organic binder having an acidic functional group;
at least one selected from the group consisting of a polyvalent metal and a polyvalent metallic compound; and
an anion-adsorptive substance having the property of adsorbing an anion of the organic binder.
Another preferred embodiment of the present invention provides a green sheet (hereinafter referred to as xe2x80x9cthe invented green sheetxe2x80x9d) obtained by molding a slurry into a sheet, which slurry includes a mixture of:
an organic binder having an acidic functional group;
a polyvalent metallic compound; and
an anion-adsorptive substance having the property of adsorbing an anion of the organic binder.
The invention provides, in a further aspect, a multilayer substrate obtained by laminating the invented paste composition through thick film screen printing and firing the laminate.
The invention provides, in yet another aspect, a multilayer substrate obtained by laminating and firing the invented green sheet.
The invented paste composition, in which an anion-adsorptive substance is incorporated into a paste composition containing an organic binder having an acidic functional group and a polyvalent metal and/or a polyvalent metallic compound, can inhibit the gelation in a mixture of the organic binder and the polyvalent metal and/or polyvalent metallic compound and has a satisfactory storage stability.
The invented green sheet, in which an anion-adsorptive substance is incorporated into a slurry containing an organic binder having an acidic functional group and a polyvalent metallic compound, can inhibit the gelation in a mixture of the organic binder and the polyvalent metallic compound such as a glass material or ceramic material and has an excellent storage stability and a satisfactory reliability after firing.
The invented multilayer substrate has, for example, its conductive layer or insulating layer formed by the invented paste composition or has its insulating layer formed by the invented green sheet. The invented multilayer substrate is therefore capable of highly precisely forming, for example, a wiring pattern or a via hole in a high density and is excellent in strength and other characteristics after firing.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.